The invention is particularly valuable in hazardous environments although it is not restricted to such use. Important users of telemetry include the Gas, Oil, Chemical and Petrochemical industries, all of which are required to meet high standards of safety and to work within stringent safety regulations.
Thus, it is a well known problem to effect remote monitoring of pressures and flows of flammable fluids without the introduction of electrical or mechanical energy of magnitude sufficient to produce a spark. There are several conventional methods of overcoming this problem, of which the two most commonly used are as follows:
(1) Instrumentation contained in explosion-proof cases.
(2) Intrinsically safe barrier circuits.
The explosion proof case is designed to contain any source of ignition and an explosion without affecting the surrounding atmosphere. The complete instrument, including the case, must be approved for use in hazardous areas by the appropriate national authority and typically has to display a reference to the certificate of approval. Such instruments are very expensive.
The intrinsically safe barrier circuit is designed to limit the electrical power to a level below that required to produce a spark. Barrier circuits must be approved by the approving authority for use in a safe area with a continuous cable, having no in-line connectors, to the instrument located in the hazardous area. Approved barrier circuits are encapsulated and contain inaccessible fuses rated at 50 mA maximum, thus an open-circuit fuse necessitates replacement of the barrier circuit. Also, the internal resistance of these barrier circuits prohibits the use of certain instruments.
The instrument associated with the barrier circuit must not possess any form of electrical energy storage of magnitude greater than the safety limit and must be approved by the authority for use in a hazardous area in conjunction with an approved intrinisically safe barrier circuit. The instrument case must not consist of any materials which would produce a spark when brought into contact with other solid materials; e.g. the dropping of tools on to the case. Other problems encountered with this type of installation include the provision of a mains isolating transformer and a separate station earth to which the barrier circuits must be connected.
The object of the present invention is to provide a system which is inherently safer than known systems, leads to substantial cost savings and can remove many of the limitations imposed by known systems.